Antenna



y 1956 w. v. FOLEY EIAL 2,756,419

ANTENNA Filed Jan. 5, 1952 WE H t 'IIIIII 7 g INVENTOR5 J WILL/HM 1/. FOLEY,

w oa/wqzp A. 5x01 77154,

ByQ gbzw ANTENNA William V. Foley and Donald R. Barthel, Baltimore, Md, assignors to The Glen L. Martin Company, Middle River, Md, a corporation of Maryland Application January 5, 1952, Serial No. 265,118

7 Claims. (Cl. 343708) This invention relates to antennae for the reception and transmission of high frequency signal energy and more particularly to such an antenna for use in an aircraft or like vehicle.

It is obviously advantageous to mount an aircraft antenna of the type described so that its exposed surface is flush with the exterior of the aircraft, thereby minimizing interference with the airflow over the aircraft surface. However, since the physical conformation of a particular antenna is dictated by the field pattern and frequency response characteristics desired, and since these characteristics may also be affected by the manner in which the antenna is mounted, previous practice has been largely confined to external antenna mountings, usually covered by a radome or other protuberance for decreasing the drag. Because of these limitations it has not previously been possible to provide a flush mounted antenna which would give wide coverage and receive a broad range of frequencies while having a substantially constant field pattern without nulls.

The antenna contemplated by this invention will provide essentially uniform coverage of a wide area, having a field pattern which has neither nulls nor any substantial depressions, and will receive a wide range of frequencies with no significant change in the pattern. At the same time, the physical conformation of the antenna will allow it to be mounted so that its exposed portion is flush with the exterior surface of the aircraft or other vehicle in which it is carried, thereby eliminating the necessity for a protruding radome on the surface of the vehicle.

It is an object of this invention, therefore, to provide an antenna which will give broad coverage, having a field pattern which is relatively high and wide.

It is another object of this invention to provide an antenna having a field pattern which is free from nulls or substantial depressions.

It is a further object of this invention to provide an antenna which will receive a relatively wide range of frequencies.

It is a still further object of this invention to provide an antenna having the above described characteristics which can be mounted flush with the exterior surface of an aircraft.

These and further objects of this invention will become apparent from the following description taken in connection with the accompanying drawings.

In the drawings:

Fig. 1 is a view of the antenna mounted at the surface of a vehicle, showing the exposed portion of the antenna.

Fig. 2 is a sectional view taken along line 2-2 of Fig. 1 showing the configuration of the antenna ridges and the method of mounting the coaxial fitting used for coupling the antenna to the feed line.

Fig. 3 is a sectional view taken along line 33 of Fig. 1.

Fig. 4 is a sectional view taken along line 44 of Fig. 3, showing the cross-sectional shape of the wave guide portion of the antenna and of the ridges.

nitecl States Patent As shown in Fig. 2, the antenna contemplated by this invention comprises a section of wave guide 1, square in cross-section, and closed at one end 2. A flange 3, extending radially from the mouth 4 of the wave guide, is provided for mounting the antenna at the surface 5 of an aircraft or other vehicle, the flange and the surface then acting as the ground plane of the antenna. To obtain the desired broadband match and field pattern characteristics, the wave guide is electrically loaded with two ridges 6 and 7, spaced from the closed end 2 of the wave guide and extending longitudinally along opposite sides thereof. As shown in Figures 1 and 4 these ridges lie in a common plane, which in the case illustrated coincides with the longitudinal median plane of the guide.

Because of the electrical discontinuity with resulting reflection which would otherwise exist at the mouth 4, it is necessary to provide means for giving a smooth transition of impedance from that of the wave guide to that of free space. To obtain the desired impedance transition characteristics, the ridges 6 and 7 are extended beyond the mouth of the wave guide curving outwardly from the center thereof to merge smoothly into the surface of the flange 3. So that the ridges will not project beyond the ground plane and into the airstream, flange 3 may be dished rearwardly slightly as at 7' to form a shallow widely flaring depression 8, thus allowing the faces 9 and it) of the ridges 6 and 7 respectively to lie in the plane of the flange 3.

It is important that the included angle between the sloping walls of the dished portion 7 of the member 3 be sufficiently great and the depression suficiently shallow to prevent this portion from acting as a horn rather than as part of the ground plane. As can be seen in Figs. 1 to 3, the size of the included angle is in the vicinity of and the shallowness of the depression 8 is such that the plane area enclosed by a line defining the outer limit of the dished portion 7 is significantly less than four times the over-all cross-sectional area of the wave guide 1. While an exact limiting value for this ratio has not been determined, it is estimated that 4:1 is a safe value and that antennas constructed with any ratio less than this will operate eflfectively on the ground plane principle in accordance with the teachings herein set forth, and not as antennas of the horn type. With the parts proportioned substantially as shown, the radiation pattern is dictated primarily by the dimensions across the open end 4 of the wave guide whereas if the portion 7' were dished too deeply so that the area ratio defined above becomes significantly greater than four, the pattern would be determined by the dimensions across the open face of this dished portion in the manner of a horn.

The immediately preceding description serves to distinguish an arrangement providing a ground plane type of operation from that in which a horn effect appears, and the expression ground plane when used in this specification and the subjoined claims includes all such transverse surfaces used in connection with wave guide antenna mouths whether smooth and regular or moderately dished within the ranges described above.

The sides of the ridges 6 and 7 converge smoothly as they approach the mouth 4 of the wave guide to join the smaller portions of the ridges which extend-beyond the mouth. As may be seen in Fig. 2, the opposing faces of the ridges within the wave guide 1 diverge as they approach the mouth 4 so that the space 11 between the ridges is wider at the mouth than elsewhere within the waveguide. The above described confi uration of the ridges allows a wide frequency coverage, and transforms the impedance of the wave guide to that of free space with no electrical discontinuities in the antenna, thus substantially eliminating reflection at the mouth 4, and giving the antenna eflicient power transfer characteristics.

To feed the antenna, a fitting 12 is provided for connecting a coaxial feed line to one side of the wave guide 1, the center conductor 13 being insulated from the metallic portion of the fitting by the dielectric portion 14, and extending thru the cylindrical passage 15 in the wall of the wave guide 1 to terminate in ridge 6.

The antenna contemplated by this invention will provide essentially uniform coverage within a beam having a half power beam width of approximately 180 in the E plane (the plane common to ridges 6 and 7) and approximately 50 in the H plane, and will receive frequencies over a range of 2.6 to 1 without any appreciable change in the field pattern, which has neither nulls nor depressions greater than 3 decibels.

The physical conformation of the antenna will allow it to be mounted so that its exposed portion is flush with the exterior surface of the aircraft or other vehicle in which it is carried, thereby eliminating the necessity for a protruding radonie on the surface of the vehicle.

While but one embodiment of this invention has been shown and described, it is to be understood that the invention is not confined to the precise details of construction herein set forth, as it is apparent that changes and modifications may he made without departing from the scope of the invention, and no limitation is intended by the phraseology of the foregoing description or illustrations in the accompanying drawings.

We claim as our invention:

1. An antenna comprising a portion of hollow wave guide open at its forward end, means forming a ground plane across the forward end of the wave guide, and a pair of ridges lying in a common plane and projecting inwardly from diametrically opposed inner walls of said wave guide, said ridges extending lengthwise along said walls of said wave guide and then laterally outwardly in opposite directions across the face of said ground plane, the forward surfaces of said ridges being coplanar with said ground plane.

2. An antenna comprising a portion of hollow wave guide of substantially constant cross-section having its rearward end closed and its forward end open, means forming a ground plane across the forward end of the wave guide, and a pair of ridges lying in a common plane and projecting inwardly from diametrically opposed inner walls of said waveguide, said ridges extending from a point forward of said closed end of said wave guide lengthwise along said walls of said wave guide and then laterally outwardly in opposite direction along said ground plane, the forward surfaces of said ridges merging with said ground plane at the outer extremities of said ridges, and said ridges being coplanar with said ground plane.

3. An antenna comprising means forming a ground plane, said ground plane having a portion thereof dished rearwardly to form a relatively shallow, forwardly facing depression in its surface, said depression having an opening at its center, a portion of hollow wave guide of substantially constant cross-section disposed with its longitudinal axis substantially normal to said ground plane, the rearward end of said wave guide being closed, and the forward end thereof being open and joined to said ground plane so that said open end coincides with the opening in the center of said depression, and a pair of ridges lying in a common plane and projecting inwardly from diametrically opposed inner walls of said wave guide, said ridges extending lengthwise along the walls of said waveguide to the forward end thereof, and then laterally outwardly in opposite directions along the sides of said depression to the outer edge thereof, the portions of said ridges extending along the sides of said depression tapering in height so that the forward sides of said ridges are '00- planar with the undished portions of said ground plane.

4. An antenna comprising means forming a ground plane, said ground plane having a portion thereof dished rearwardly to form a forwardly facing, shallow depression in its surface, said depression having an opening at its center, a portion of hollow wave guide of substantially constant cross-section having its forward end attached to the rearward side of said ground plane means so that the opening in the forward end of said wave guide coincides with the opening in said depression, and a pair of ridges lying in a common plane and projecting inwardly from diametrically opposite inner walls thereof, said ridges extending lengthwise along said wave guide to the forward end thereof and then laterally outwardly in opposite directions to the outer edge of said depression, the forward faces of the portions of the ridges lying within said depression being coplanar with the undished portions of said ground plane.

5. An antenna comprising means forming a ground plane, said ground plane having a portion thereof dished rearwardly to form a relatively shallow, forwardly facing depression in its surface, said depression having an opening at its center, a portion of wave guide of substantially constant cross-section closed :at its rearward end by a wall perpendicular to the longitudinal axis of said wave guide, and having its forward end joined to the rearward side of said recessed portion of said ground plane means so that the opening in said forward end of said wave guide coincides with the opening insaid depression, and a pair of ridges lying in a common plane and projecting inwardly from diametrically opposite inner walls of said waveguide, said ridges extending lengthwise along the walls of said waveguide and then laterally outwardly in opposite directions along the sides of said depression to the outer edge thereof, the portions of said ridges lying within said depression tapering in thickness so that no portion of said ridges extends forward of the undished portion of said ground plane.

6. An antenna comprising means forming a ground plane, said ground plane having a portion thereof dished rearwardly to form a relatively shallow, widely flaring, generally conical, forwardly facing depression therein, said depression having an opening at its center, a portion of.

waveguide of substantially constant cross-section closed at its rearward end by a wall perpendicular to the longitudinal axis of said waveguide, and having its forward end joined to the rearward side of said recessed portion of said ground plane means so that the opening in the forward end of said wave guide coincides with the opening in said depression, and a pair of ridges lying in a common plane and projecting inwardly from diametrically opposite inner walls of said wave guide, said ridges extending from v a point forward of said rear wall of said wave guide lengthwise along the walls of said wave guide and thenlaterally outwardly in opposite directions along the sides of said depression to the outer edge thereof, the portions of said ridges lying within said waveguide being of greater cross-section than the portions lying along the sides of said depression, and the portions lying along said depression tapering in height so that no part of said ridges lies forward of the undished portion of said ground plane.

7. An antenna comprising means forming a ground plane, said ground plane having a portion thereof dished rearwardly to'form a relatively shallow, generally conical, forwardly facing depression therein, said depression having a rectangular opening in its center, a portion of wave guide of constant rectangular cross-section closed at its rearward end by a wall perpendicular to the longitudinal axis of said wave guide, and having its forward end joined to the rearward side of said recessed portion of said ground plane means so that the longitudinal axis of said wave guide is perpendicular to said ground plane and the opening in the forward end of said wave guide coincides with the opening in said depression, a pair of ridges, rectangular in cross-section, lying in a common plane and projecting inwardly from diametrically opposed inner walls of said wave guide, said ridges extending from a point forward of said rear wall of said wave guide lengthwise along the walls of said wave guide and then laterally outwardly along the sides of said depression to tions of the ridges lying along the sides of said depression varying in height so that the forward faces of said ridges are coplanar with the undished portion of said ground plane, and means for connecting a source of high frequency energy to said antenna.

References Cited in the file of this patent UNITED STATES PATENTS 2,343,531 Buchholz Mar. 7, 1944 6 Kees Nov. 18, 1947 Riblet July 29, 1952 Alford Sept. 23, 1952 Alford Nov. 10, 1953 OTHER REFERENCES Microwave Transmission Circuits (vol. 9, of M. I. T. Radiation Laboratory Series), George L. Ragan, copyright May 21, 1948, McGraw-Hill Book Company, New

10 York; pages 358, 360 and 361. 

